© D
HI
For areas located near the coast, flooding can have a long-term impact on
homes and businesses. For the counties around the San Francisco Bay,
detailed information about which areas are prone to flooding is vital.
Unfortunately, the available information to identify flood prone areas was non-
existent or outdated. As such, the United States Federal Emergency
Management Agency (FEMA) asked us to examine the flood risks for the entire
Bay Area. Using MIKE 21, we developed two-dimensional (2D) regional scale
coastal models of the San Francisco Bay shoreline. The data from our models
provided more robust and reliable region-wide information for flood risk
analysis. This enabled local governments to improve their flood management
practices. It also gave the people living and working around the Bay the
information they need to protect themselves better against floods.
DETERMINING SAN FRANCISCO BAY FLOOD RISKS
Located on the West Coast of the United States, the shores of the San Francisco
Bay are home to more than seven million people. The scenic but low-lying bay front
areas are home to the corporate headquarters of Google, Facebook, Cisco, Yahoo!
and Apple. With billions of dollars of infrastructure located adjacent to the hundreds
of miles of shoreline, an accurate assessment of the flood risk is vital. Extreme
water levels caused by storms and other factors can have serious implications for
the people and businesses that call the Bay Area home.
SUMMARY
CLIENT
U.S. Federal Emergency Management Agency (FEMA), Region IX
CHALLENGE
Lack of knowledge about how flood risk has changed in San Francisco Bay since the last regional study performed in 1983
SOLUTION
Two dimensional (2D) modelling to examine flood risks for the entire San Francisco Bay Area
VALUE
Increased confidence in the knowledge of flood prone areas for coastal communities around San Francisco Bay
Ability to improve floodplain management practices for counties in the Bay Area
LOCATION / COUNTRY
San Francisco Bay, California, USA
The Golden Gate Bridge crosses the opening of the San Francisco Bay where it meets the
Pacific Ocean. ©Dale Kerper
DHI CASE STORY
FLOOD RISK – PROTECTING COASTAL COMMUNITIES
Using 2D modelling to improve San Francisco Bay’s floodplain maps
COAST & MARINE
The Bay Area has not had a comprehensive region-wide study
since the early 1980s. Although innovative for its time, the
previous study had several shortcomings. It relied on a limited
number of tide gages that did not take measurements for a
long enough period of time (with the exception of the Golden
Gate Bridge tide gage). It also did not take into account the
effect of waves. In addition, the study was inaccessible as the
paper records had been archived.
To obtain a better idea of coastal flood hazards for the entire
San Francisco Bay, FEMA asked us to perform a regional
Flood Insurance Study (FIS). The FIS – a report that
examines the flood risks of a specific area – is based on data
collected from:
statistical analyses of past river flow, storm tides
consultations with potentially affected communities
hydrographic bathymetric surveys
floodplain topographic surveys (LiDAR for example)
wave height measurements
wind measurements
hydrologic and hydraulic analyses
MODELLING THE BAY AREA We utilised 2D modelling to improve upon the methods that
were available in the previous study. Using MIKE 21, we
developed a system of regional scale coastal models in order
to predict the following for the entire San Francisco Bay:
extreme tide (astronomical tide plus storm surge)
swell entering from the Pacific Ocean
sea waves generated from local winds in the Bay
Normally, we would apply a highly detailed study to a focused
study area. This project, however, required the same level of
detail for the entire 250 mile-long Bay Area shoreline. Several
factors contribute to the variability and complexity of the Bay
shoreline geography, including:
orientation of the shoreline
exposure to wind and waves (sheltering for example)
water depths
regional/climactic variation in wind and wave conditions
(micro-climates)
Because of these factors, it was very difficult to identify
singular storm events that would apply equally across the
entire Bay Area. To allow us to identify the storms in advance,
we operated the models in hindcast mode to produce
continuous results of water levels and waves covering:
31 years in the North and Central Bay
54 years in the South Bay
Following this, we used the regional model results to establish
maximum 1% (100-year) and 0.2% (500-year) Still Water
Levels (SWLs) – the maximum water level that will occur –
and wave height conditions. We then utilised this information
to drive local response-based models of:
water level
wave setup
wave run-up
overland waves
We calibrated and validated the models using National
Oceanic and Atmospheric Administration (NOAA) tide gage
measurements from around the bay, as well as various wave
measurements from inside and outside the bay.
UPDATING FLOODPLAIN MAPS
The results of the regional modelling study were key
components for the development of FIS floodplain maps for
the entire Bay Area. FEMA and its study contractors are using
the regional model results as boundary conditions for further
detailed coastal hazard analysis. This will also be the basis for
most future studies.
The detailed coastal hazard analysis could have far reaching
implications for the businesses and people who live and work
around the Bay. The results could lead to a modification in
flood risk designations for coastal communities, which could in
turn lead to changes in:
flood insurance rates
land-use planning around the Bay
floodplain management practices
By having a clearer idea of where flooding is most likely to
occur, communities surrounding San Francisco Bay can take
the appropriate measures to ensure their safety.
DHI CASE STORY ©
DH
I
A time snapshot of the MIKE 21 regional model result, showing con-
tours and vectors of currents for a flood tide condition during the his-
torical storm event of 1983. © DHI
Contact: Dale Kerper - [email protected]
For more information visit: www.dhigroup.com
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